Intrathymic Glucocorticoid Production and Thymocyte Survival: Another Piece in the Puzzle
http://www.100md.com
内分泌学杂志 2005年第6期
Department of Biochemistry and Molecular Biology (T.J.C., D.R.L.), Monash University, Clayton, Victoria 3800, Australia; and Department of Microbiology and Immunology (D.R.L., D.I.G.), University of Melbourne, Parkville, Victoria 3010, Australia
Address all correspondence and requests for reprints to: T. J. Cole, Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria 3800, Australia. E-mail: tim.cole@med.monash.edu.au.
It is well accepted that T cells, particularly immature thymocytes, are very sensitive to glucocorticoids, with an elevation of glucocorticoid levels during stress causing profound involution of the thymus (1). Despite this, the significance of glucocorticoids in thymocyte development and function remains an unclear and controversial area. A report in this issue of Endocrinology (2) has again turned the spotlight on the potential importance of glucocorticoids synthesized by the thymus, most likely arising from thymic epithelial cells. The authors present results of a study where a conditional rat glucocorticoid receptor (GR) transgene has been specifically expressed in mouse T cells using the tetracycline-inducible expression system with a human T cell-specific CD2 (hCD2) promoter. They show that in adrenalectomized (to exclude systemic glucocorticoids) transgenic mice, there is a dramatic increase in thymocyte death by apoptosis after doxycycline-induction of the GR transgene, implicating induction of T-cell death from endogenously produced glucocorticoids. This was replicated in vitro using a thymic organ culture system and the increased death in vivo and in vitro could be at least partially blocked by the GR antagonist RU486. These results give compelling evidence for the presence of thymic-produced glucocorticoids that, at least in this transgenic system, are able to trigger significant thymocyte apoptosis. This clearly supports the concept that the thymus is a source of functionally significant levels of glucocorticoid hormone, although a number of unanswered questions remain.
Several papers have reported the presence or production of endogenous glucocorticoids in the thymus, most likely by thymic stromal cells (3, 4, 5, 6, 7, 8, 9). The synthesis of glucocorticoids in the thymus was initially suggested in studies using in vitro fetal thymic organ culture with the use of selective inhibitors of corticosteroid biosynthesis, such as metyrapone (3, 4, 5, 6). The capacity of the thymus to synthesize endogenous glucocorticoids was then suggested by the detection of the complement of glucocorticoid biosynthetic enzyme genes in the mouse and chicken thymus by RT-PCR, and chromatographic analysis of steroid production from thymus and thymic epithelial cell line extracts, although enzymatic activities in comparison to the adrenal were low (7, 8). Blockade of glucocorticoid production in the thymus appeared to cause deletion of CD4+CD8+ thymocytes. These studies suggested that GR signaling influenced T-cell receptor (TCR)-mediated antigen-specific thymocyte development and survival, primarily by antagonizing TCR signaling and thereby inhibiting TCR-dependent thymocyte deletion. This hypothesis was subsequently supported by other studies using GR antisense transgenic mice to reduce GR expression (10). In each case, reduced GR signaling resulted in reduced thymocyte numbers, suggesting that physiological levels of GR signaling is an important factor in thymocyte survival, associated with intrathymic-positive selection (11).
Despite this compelling evidence from multiple models, other groups with different models have failed to generate data to support a role for glucocorticoids in promoting thymocyte survival. Some other studies have suggested that glucocorticoids promote thymocyte death and that blockade of glucocorticoid signaling enhances survival, whereas others, using GR knockout or mutant mice, have failed to show an influence of glucocorticoid signaling on thymocyte development, selection, or survival (12, 13, 14, 15, 16). Clearly this remains an area of ongoing controversy that requires further investigation (17, 18, 19, 20, 21, 22, 23). Although the precise influence of glucocorticoid hormones in the thymus remains unclear, the exquisite sensitivity of thymocytes to glucocorticoids, and the constitutive production of glucocorticoid hormones in this tissue, strongly suggests that this signaling pathway can at least potentially play a role in T-cell development. This is obvious at times of stress, where glucocorticoid levels increase leading to massive thymocyte apoptosis (1), and it is possible that glucocorticoids might normally play a more subtle role in the thymus that is not always detectable or reproducible across different mouse models of T-cell development. For example, glucocorticoids have recently been showed to strongly activate the IL7-R--chain on T cells, and endogenous glucocorticoids may play a regulatory role in this activation (24).
The present study using adrenalectomized T cell-specific-inducible GR transgenic mice implicates the action of endogenously produced glucocorticoids in thymocyte survival. This is a very interesting result that raises some new and important questions. Does this reflect a physiological response to thymus-derived glucocorticoid levels? This obviously depends on the relative level of GR expression from the transgenic insert, compared with endogenous wild-type GR levels. This was not directly compared in this study but would provide valuable information to help interpret these data. One experiment in this study (see Fig. 5 in Ref. 2) appears to show that thymus-derived glucocorticoids are responsible for reduced thymocyte numbers even when the GR transgene is not induced by doxycycline. Treatment of adrenalectomized hCD2-GR transgenic mice (apparently without doxycycline-mediated induction of this transgene) with the GR antagonist RU486 resulted in increased thymocyte numbers. This is perhaps the best evidence for a physiological role for thymic-derived glucocorticoid production in controlling thymocyte numbers in vivo. However, whether a similar result would be achieved in nontransgenic mice remains unknown but would seem to be an important experiment in this context. Other interesting questions raised by this study include how active is the glucocorticoid biosynthetic pathway in the thymus and whether it undergoes hypothalamic-pituitary regulatory control, similar to the adrenal cortex.
In summary, this study provides further evidence for the production of intrathymic glucocorticoids. Although this does not resolve the ongoing controversy surrounding the precise function of glucocorticoids in intrathymic T-cell development, it reemphasizes the fact that glucocorticoids are produced in the thymus at functionally significant levels, which restates the question of the role of intrathymic glucocorticoid production. Determination of the response from non-GR transgenic T cells to thymus-derived glucocorticoid production is clearly an important step toward appreciating the physiological significance of the results presented in these GR transgenic mice. Such homeostatic roles may, however, be overwhelmed during times of systemic stress because high levels of adrenal-derived glucocorticoids flood the thymus and bring about classic thymocyte death and thymic involution. The endocrine community waits with great interest for further results and details on intrathymic glucocorticoid production, and its physiological role in T-cell homeostasis and development.
References
Sapolsky, RM, Romero LM, Munck AU 2000 How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions. Endocr Rev 21:55–89
Pazirandeh A, Jondal M, Okret S 2005 Conditional expression of a glucocorticoid receptor transgene in thymocytes reveals a role for thymic-derived glucocorticoids in thymopoiesis in vivo. Endocrinology 146:2501–2507
Vacchio MS, Papadopoulos V, Ashwell JD 1994 Steroid production in the thymus: implications for thymocyte selection. J Exp Med 179:1835–1846
Vacchio MS, Ashwell JD 1997 Thymus-derived glucocorticoids regulate antigen-specific positive selection. J Exp Med 185:2033–2038
Vacchio MS, Lee JY, Ashwell JD 1999 Thymus-derived glucocorticoids set the thresholds for thymocyte selection by inhibiting TCR-mediated thymocyte activation. J Immunol 163:1327–1333
Pazirandeh A, Xue Y, Rafter I, Sjovall J, Jondal M, Okret S 1999 Paracrine glucocorticoid activity produced by mouse thymic epithelial cells. FASEB J 13:893–901
Lechner O, Wiegers GJ, Oliveira-Dos-Santos AJ, Dietrich H, Recheis H, Waterman M, Boyd R, Wick G 2000 Glucocorticoid production in the murine thymus. Eur J Immunol 30:337–346
Lechner O, Dietrich H, Wiegers GJ, Vacchio M, Wick G 2001 Glucocorticoid production in the chicken bursa and thymus. Eur J Immunol 13:769–776
Jenkinson EJ, Parnell S, Shuttleworth J, Owen JJ, Anderson G 1999 Specialized ability of thymic epithelial cells to mediate positive selection does not require expression of the steroidogenic enzyme p450scc. J Immunol 163:5781–5785
King LB, Vacchio MS, Dixon K, Hunziker R, Margulies DH, Ashwell JD 1995 A targeted glucocorticoid receptor antisense transgene increases thymocyte apoptosis and alters thymocyte development. Immunity 3:647–656
Ashwell JD, Lu FW, Vacchio MS 2000 Glucocorticoids in T cell development and function. Annu Rev Immunol 18:309–345
Reichardt HM, Kaestner KH, Tuckermann J, Kretz O, Wessely O, Bock R, Gass P, Schmid W, Herrlich P, Angel P, Schutz G 1998 DNA binding of the glucocorticoid receptor is not essential for survival. Cell 93:531–541
Purton JF, Boyd RL, Cole TJ, Godfrey DI 2000 Intrathymic T cell development and selection proceeds normally in the absence of glucocorticoid receptor signaling. Immunity 13:179–186
Purton JF, Zhan Y, Liddicoat DR, Hardy CL, Lew AM, Cole TJ and Godfrey DI 2002 Glucocorticoid receptor deficient thymic and peripheral T cells develop normally in adult mice. Eur J Immunol 32:3546–3555
Brewer JA, Kanagawa O, Sleckman BP, Muglia LJ 2002 Thymocyte apoptosis induced by T cell activation is mediated by glucocorticoids in vivo. J Immunol 169:1837–1843
Brewer JA, Khor B, Vogt SK, Muglia LM, Fujiwara H, Haegele KE, Sleckman BP, Muglia LJ 2003 T-cell glucocorticoid receptor is required to suppress COX-2-mediated lethal immune activation. Nat Med 9:1318–1322
Ashwell JD, Vacchio MS, Galon J 2000 Do glucocorticoids participate in intrathymic T-cell development? Immunol Today 21:644–645
Godfrey DI 2000 Reply to Ashwell, Vacchio and Galon. Immunol Today 21:645–646
Godfrey DI, Purton JF, Boyd RL, Cole TJ 2001 Glucocorticoids and the thymus: the view from the middle of the road. Trends Immunol 22:243
Godfrey DI, Purton JF, Boyd RL, Cole TJ 2001 Stress-free T cell development: glucocorticoids are not obligatory. Immunol Today 21:606–611
Jondal M, Pazirandeh A, Okret S 2001 Role for glucocorticoids in the thymus? Trends Immunol 22:185–186
Mittelstadt PR, Ashwell JD 2003 Disruption of glucocorticoid receptor exon 2 yields a ligand-responsive C-terminal fragment that regulates gene expression. Mol Endocrinol 17:1534–1542
Jondal M, Pazirandeh A, Okret S 2004 Different roles for glucocorticoids in thymocyte homeostasis? Trends Immunol 25:595–600
Franchimont D, Galon J, Vacchio MS, Fan S, Visconti R, Frucht DM, Geenen V, Chrousos GP, Ashwell JD, O’Shea JJ 2002 Positive effects of glucocorticoids on T cell function by up-regulation of IL-7 receptor . J Immunol 168:2212–2218(Timothy J. Cole, Douglas )
Address all correspondence and requests for reprints to: T. J. Cole, Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria 3800, Australia. E-mail: tim.cole@med.monash.edu.au.
It is well accepted that T cells, particularly immature thymocytes, are very sensitive to glucocorticoids, with an elevation of glucocorticoid levels during stress causing profound involution of the thymus (1). Despite this, the significance of glucocorticoids in thymocyte development and function remains an unclear and controversial area. A report in this issue of Endocrinology (2) has again turned the spotlight on the potential importance of glucocorticoids synthesized by the thymus, most likely arising from thymic epithelial cells. The authors present results of a study where a conditional rat glucocorticoid receptor (GR) transgene has been specifically expressed in mouse T cells using the tetracycline-inducible expression system with a human T cell-specific CD2 (hCD2) promoter. They show that in adrenalectomized (to exclude systemic glucocorticoids) transgenic mice, there is a dramatic increase in thymocyte death by apoptosis after doxycycline-induction of the GR transgene, implicating induction of T-cell death from endogenously produced glucocorticoids. This was replicated in vitro using a thymic organ culture system and the increased death in vivo and in vitro could be at least partially blocked by the GR antagonist RU486. These results give compelling evidence for the presence of thymic-produced glucocorticoids that, at least in this transgenic system, are able to trigger significant thymocyte apoptosis. This clearly supports the concept that the thymus is a source of functionally significant levels of glucocorticoid hormone, although a number of unanswered questions remain.
Several papers have reported the presence or production of endogenous glucocorticoids in the thymus, most likely by thymic stromal cells (3, 4, 5, 6, 7, 8, 9). The synthesis of glucocorticoids in the thymus was initially suggested in studies using in vitro fetal thymic organ culture with the use of selective inhibitors of corticosteroid biosynthesis, such as metyrapone (3, 4, 5, 6). The capacity of the thymus to synthesize endogenous glucocorticoids was then suggested by the detection of the complement of glucocorticoid biosynthetic enzyme genes in the mouse and chicken thymus by RT-PCR, and chromatographic analysis of steroid production from thymus and thymic epithelial cell line extracts, although enzymatic activities in comparison to the adrenal were low (7, 8). Blockade of glucocorticoid production in the thymus appeared to cause deletion of CD4+CD8+ thymocytes. These studies suggested that GR signaling influenced T-cell receptor (TCR)-mediated antigen-specific thymocyte development and survival, primarily by antagonizing TCR signaling and thereby inhibiting TCR-dependent thymocyte deletion. This hypothesis was subsequently supported by other studies using GR antisense transgenic mice to reduce GR expression (10). In each case, reduced GR signaling resulted in reduced thymocyte numbers, suggesting that physiological levels of GR signaling is an important factor in thymocyte survival, associated with intrathymic-positive selection (11).
Despite this compelling evidence from multiple models, other groups with different models have failed to generate data to support a role for glucocorticoids in promoting thymocyte survival. Some other studies have suggested that glucocorticoids promote thymocyte death and that blockade of glucocorticoid signaling enhances survival, whereas others, using GR knockout or mutant mice, have failed to show an influence of glucocorticoid signaling on thymocyte development, selection, or survival (12, 13, 14, 15, 16). Clearly this remains an area of ongoing controversy that requires further investigation (17, 18, 19, 20, 21, 22, 23). Although the precise influence of glucocorticoid hormones in the thymus remains unclear, the exquisite sensitivity of thymocytes to glucocorticoids, and the constitutive production of glucocorticoid hormones in this tissue, strongly suggests that this signaling pathway can at least potentially play a role in T-cell development. This is obvious at times of stress, where glucocorticoid levels increase leading to massive thymocyte apoptosis (1), and it is possible that glucocorticoids might normally play a more subtle role in the thymus that is not always detectable or reproducible across different mouse models of T-cell development. For example, glucocorticoids have recently been showed to strongly activate the IL7-R--chain on T cells, and endogenous glucocorticoids may play a regulatory role in this activation (24).
The present study using adrenalectomized T cell-specific-inducible GR transgenic mice implicates the action of endogenously produced glucocorticoids in thymocyte survival. This is a very interesting result that raises some new and important questions. Does this reflect a physiological response to thymus-derived glucocorticoid levels? This obviously depends on the relative level of GR expression from the transgenic insert, compared with endogenous wild-type GR levels. This was not directly compared in this study but would provide valuable information to help interpret these data. One experiment in this study (see Fig. 5 in Ref. 2) appears to show that thymus-derived glucocorticoids are responsible for reduced thymocyte numbers even when the GR transgene is not induced by doxycycline. Treatment of adrenalectomized hCD2-GR transgenic mice (apparently without doxycycline-mediated induction of this transgene) with the GR antagonist RU486 resulted in increased thymocyte numbers. This is perhaps the best evidence for a physiological role for thymic-derived glucocorticoid production in controlling thymocyte numbers in vivo. However, whether a similar result would be achieved in nontransgenic mice remains unknown but would seem to be an important experiment in this context. Other interesting questions raised by this study include how active is the glucocorticoid biosynthetic pathway in the thymus and whether it undergoes hypothalamic-pituitary regulatory control, similar to the adrenal cortex.
In summary, this study provides further evidence for the production of intrathymic glucocorticoids. Although this does not resolve the ongoing controversy surrounding the precise function of glucocorticoids in intrathymic T-cell development, it reemphasizes the fact that glucocorticoids are produced in the thymus at functionally significant levels, which restates the question of the role of intrathymic glucocorticoid production. Determination of the response from non-GR transgenic T cells to thymus-derived glucocorticoid production is clearly an important step toward appreciating the physiological significance of the results presented in these GR transgenic mice. Such homeostatic roles may, however, be overwhelmed during times of systemic stress because high levels of adrenal-derived glucocorticoids flood the thymus and bring about classic thymocyte death and thymic involution. The endocrine community waits with great interest for further results and details on intrathymic glucocorticoid production, and its physiological role in T-cell homeostasis and development.
References
Sapolsky, RM, Romero LM, Munck AU 2000 How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions. Endocr Rev 21:55–89
Pazirandeh A, Jondal M, Okret S 2005 Conditional expression of a glucocorticoid receptor transgene in thymocytes reveals a role for thymic-derived glucocorticoids in thymopoiesis in vivo. Endocrinology 146:2501–2507
Vacchio MS, Papadopoulos V, Ashwell JD 1994 Steroid production in the thymus: implications for thymocyte selection. J Exp Med 179:1835–1846
Vacchio MS, Ashwell JD 1997 Thymus-derived glucocorticoids regulate antigen-specific positive selection. J Exp Med 185:2033–2038
Vacchio MS, Lee JY, Ashwell JD 1999 Thymus-derived glucocorticoids set the thresholds for thymocyte selection by inhibiting TCR-mediated thymocyte activation. J Immunol 163:1327–1333
Pazirandeh A, Xue Y, Rafter I, Sjovall J, Jondal M, Okret S 1999 Paracrine glucocorticoid activity produced by mouse thymic epithelial cells. FASEB J 13:893–901
Lechner O, Wiegers GJ, Oliveira-Dos-Santos AJ, Dietrich H, Recheis H, Waterman M, Boyd R, Wick G 2000 Glucocorticoid production in the murine thymus. Eur J Immunol 30:337–346
Lechner O, Dietrich H, Wiegers GJ, Vacchio M, Wick G 2001 Glucocorticoid production in the chicken bursa and thymus. Eur J Immunol 13:769–776
Jenkinson EJ, Parnell S, Shuttleworth J, Owen JJ, Anderson G 1999 Specialized ability of thymic epithelial cells to mediate positive selection does not require expression of the steroidogenic enzyme p450scc. J Immunol 163:5781–5785
King LB, Vacchio MS, Dixon K, Hunziker R, Margulies DH, Ashwell JD 1995 A targeted glucocorticoid receptor antisense transgene increases thymocyte apoptosis and alters thymocyte development. Immunity 3:647–656
Ashwell JD, Lu FW, Vacchio MS 2000 Glucocorticoids in T cell development and function. Annu Rev Immunol 18:309–345
Reichardt HM, Kaestner KH, Tuckermann J, Kretz O, Wessely O, Bock R, Gass P, Schmid W, Herrlich P, Angel P, Schutz G 1998 DNA binding of the glucocorticoid receptor is not essential for survival. Cell 93:531–541
Purton JF, Boyd RL, Cole TJ, Godfrey DI 2000 Intrathymic T cell development and selection proceeds normally in the absence of glucocorticoid receptor signaling. Immunity 13:179–186
Purton JF, Zhan Y, Liddicoat DR, Hardy CL, Lew AM, Cole TJ and Godfrey DI 2002 Glucocorticoid receptor deficient thymic and peripheral T cells develop normally in adult mice. Eur J Immunol 32:3546–3555
Brewer JA, Kanagawa O, Sleckman BP, Muglia LJ 2002 Thymocyte apoptosis induced by T cell activation is mediated by glucocorticoids in vivo. J Immunol 169:1837–1843
Brewer JA, Khor B, Vogt SK, Muglia LM, Fujiwara H, Haegele KE, Sleckman BP, Muglia LJ 2003 T-cell glucocorticoid receptor is required to suppress COX-2-mediated lethal immune activation. Nat Med 9:1318–1322
Ashwell JD, Vacchio MS, Galon J 2000 Do glucocorticoids participate in intrathymic T-cell development? Immunol Today 21:644–645
Godfrey DI 2000 Reply to Ashwell, Vacchio and Galon. Immunol Today 21:645–646
Godfrey DI, Purton JF, Boyd RL, Cole TJ 2001 Glucocorticoids and the thymus: the view from the middle of the road. Trends Immunol 22:243
Godfrey DI, Purton JF, Boyd RL, Cole TJ 2001 Stress-free T cell development: glucocorticoids are not obligatory. Immunol Today 21:606–611
Jondal M, Pazirandeh A, Okret S 2001 Role for glucocorticoids in the thymus? Trends Immunol 22:185–186
Mittelstadt PR, Ashwell JD 2003 Disruption of glucocorticoid receptor exon 2 yields a ligand-responsive C-terminal fragment that regulates gene expression. Mol Endocrinol 17:1534–1542
Jondal M, Pazirandeh A, Okret S 2004 Different roles for glucocorticoids in thymocyte homeostasis? Trends Immunol 25:595–600
Franchimont D, Galon J, Vacchio MS, Fan S, Visconti R, Frucht DM, Geenen V, Chrousos GP, Ashwell JD, O’Shea JJ 2002 Positive effects of glucocorticoids on T cell function by up-regulation of IL-7 receptor . J Immunol 168:2212–2218(Timothy J. Cole, Douglas )